remove some todos
add token_spec
add document_spec
don't automatically require other gems
add tests relating to other libraries
add specs for collection without docs
remove tokens attr_reader
allow setting term counts and size
add more docs
add clarification to readme
add #plain_term_frequency to Document
avoid marshalling errors
remove Document#term_frequency
rename plain_term_frequency to term_count
remove aliases
add class docs
add TermCountModel
make TermCountModel#document_count public
fix division by zero
add TfIdfModel